Light & Medium Bronze-EXPECT COLOR VARIATION!

Light and medium bronze anodized finishes can be a beautiful and lasting finish for aluminum. Specifiers often choose them because when handled correctly, they are of surpassing beauty. But because expectations can run high for these finishes, light and medium bronze can also become a nightmare for the anodizer, the specifier, and the building owner. These colors are subject to severe color variation from piece to piece. This article will attempt to explain why light and medium bronze vary in color and what you can do to prevent trouble.

With lighter colors, the human eye perceives more variations in chromaticity and hue (two key components of color). This variation is far less noticeable with darker colors. Lighter colors also develop very quickly in the anodizing tanks. Sometimes, just a few extra seconds in the tank will make a difference between a light and medium bronze color. Thus, the margin for error in anodizing these colors is smaller. While these problems are frustrating, there is a way to help. The problems with anodizing lighter colors can be minimized if the orders are handled carefully.

Anodizing color is affected by numerous variables: alloy type and temper, load time, etch time, tank temperature and coating thickness just to name a few. We are able to control many of these variables in our process. However, when finishing customers’ metal, we are unable to control the alloy type. Customers can help improve the color consistency of light and medium bronze anodized jobs through careful alloy selection. Certain types of alloys anodize better than others. For flat or fabricated sheet items, 5005 aluminum alloy produces the best results. For extrusions, 6063 is the best alloy choice.

To take alloy selection a step further, supplying metal (sheets, extrusions, fabricated parts) from one lot of material can improve color consistency. When aluminum is produced, each lot has slight differences: alloy constituents, temper, extruded temperature, grain, rolled temperature, etc. These slight differences can have a big effect on anodizing color. For example, material from one lot anodized light bronze might produce a color to the light side of a range. Whereas material anodized from another lot might produce a color near the middle of the range, while both lots were anodized using the exact same process and tank time. Thus, anodizing material from one single lot will minimize the chances of color variation. Customers often want to match the color of a piece of sheet to the color of an extrusion. Unfortunately this is not possible, especially with light and medium bronze. Different alloys take on slightly different appearances when anodized. Thus, an anodized piece of 5005 sheet will look slightly different than an anodized piece of 6063 extrusion. Furthermore, we supply our range samples on 5005 sheet alloy. If a customer is looking to match their storefront extrusion to our 5005 range samples, the color will not match.

For best results, customers should select a target of the color they want. For large job, say over 5,000 square feet, it may be pratical to let the anodizer create range samples from the lot of material they plan to anodize, thereby giving the customer a feeling for the color range they can expect. For more information see our application note on range samples.

Producing a perfect match with light or medium bronze anodizing is impossible. Because so many variables exist, a certain degree of color variation is inevitable. Given that some color variation will exist, a way to minimize the color variation is for SAF to use a target color when anodizing the job. This brings us to the dichotomy between sales and production. Production must have a target to do their job, but salespeople need something called “range samples.” Range samples are really a technique to adjust the expectations of specifiers and customers and are not helpful to the production staff. The reason is that color is just one of the aspects of appearance and not even color can be measured on a light-to-dark axis. What one might describe as “lighter,” might really be “yellower.” SAF uses colorimeters to create measureable readings of color variation. Colorimeters are at the forefront of quality control practices for both anodizing and painting.

SAF COLORIMETER

Range samples are only useful to give someone a feeling for the degree of variation that is possible. They are not scientifically defined limits of color. For more information about range samples please contact your SAF representative.

The bottom line is that SAF requires a target color on all light and medium bronze projects. For best results the next time you have a light or medium bronze anodize job, remember these key points: